Entrainment mixers are devices of commendably simple construction appropriate for incorporation in apparatus of this character. However, their simplicity also imposes constraints upon their controllability and performance under varying operational circumstances. GB-A-2174609 discusses these problems and discloses an arrangement that addresses these problems in a preferred type of resuscitation/ventilation apparatus. In summary, entrainment mixers that can provide a high mixing ratio deliver a gas mixture with flow rates and mixing ratios that are highly dependent upon the back pressure to which their output is exposed, whereas those that can only provide a low mixing ratio, typically in the range 1:1 to 2:1, are able to do so with relative stability of flow rate and mixing ratio over a wide back pressure range. However, as established in the art, resuscitation/ventilation apparatus having an entrainment mixer adapted to deliver a stable low ratio gas mixture has been arranged to deliver selectively a mixture at a single, fixed, ratio as an alternative to delivering 100% oxygen. Thus, typically, a resuscitation/ventilation apparatus with an entrainment mixer has been arranged to deliver a gas mixture at one oxygen concentration only, selected from 60%, 50% and 45%, as a single alternative to the delivery of 100% oxygen by the apparatus.